Alternate, superseded and interchangeable part numbers: what a buyer can accept

Where interchangeability is actually stated

Interchangeability is a statement made by whoever controls the design, and for a buyer it lives in a handful of documents:

• The aircraft's illustrated parts catalog (IPC, or AIPC). Listed by figure and item, with effectivity. It is the document that "may be consulted to determine if a part can be installed on a particular aircraft or engine," as one parts supplier's guide puts it.
• The component maintenance manual (CMM). For the parts inside a component, the component manufacturer's manual carries its own illustrated parts list, with its own interchangeability notations.
• Service bulletins (SB). When an SB changes a part, the parts list shows both configurations — marked (PRE SB xxxx) and (POST SB xxxx) in the CMM quoted below — and the modified part may be re-identified with a new part number.
• Airworthiness directives (AD). When an AD mandates the change, it can forbid installing the old part number outright.

Not on that list: a supplier's quote, a marketplace listing, a cross-reference database, a military stock number. Each can be a useful lead. None is the design holder's statement about your aircraft. Even the IPC is the manufacturer's document rather than FAA-approved data (the FAA's PMA order says so plainly), so how it is used to establish eligibility is set by your own approved procedures.

IPC and CMM notations: five terms, five different claims

In a component maintenance manual published by its manufacturer in ATA 100 format, interchangeability is stated in the nomenclature column of the parts list, and the manual defines the terms like this:

The same manual also lists alternates inline — a groove pin, for example, carries "(ALT – 332625)". That is the manufacturer's alternate, in the manufacturer's document. "ALT" typed on a supplier's quote line looks the same and is a different thing: the supplier's suggestion.

Airframe manufacturers' catalogs express the same ideas in their own wording and codes, defined in each catalog's introduction. Read the front matter of the IPC for your type rather than any generic list, this one included.

One-way and two-way interchangeability

Two-way interchangeability means either part can be installed in place of the other. One-way means, in the words of one maintenance-software manual, "part A can be used to replace part B, but part B cannot be used to replace part A." The direction is the whole point: a newer part can often go where the older one was, while the older part cannot go back in place of the newer.

An AD shows how hard that line can be. FAA AD 2017-13-10, for certain Airbus A319, A320 and A321 models, requires the engine fan cowl doors to be modified under Airbus Service Bulletin A320-71-1069 and re-identified with new part numbers — the left-side door 740-4000-501, for instance, becomes 740-4000-9501. After the modification, the AD says "do not install" a door carrying an old part number, and aircraft without an old-number door may not have one installed from the AD's effective date.

For a buyer, that means a quote for 740-4000-501 against a request for 740-4000-9501 is not an acceptable alternate for an aircraft the AD applies to: the old number is exactly what the AD says not to install. The other direction works: fitting a -9501 door in place of a -501 is one of the ways the AD allows compliance. That is one-way interchangeability, made mandatory by an AD. Before accepting any older or superseded number, check the SBs your fleet has embodied and the ADs that apply to it.

Dash numbers: -3 is not automatically -5

The FAA's maintenance technician handbook explains how dash numbers arise: when more than one detail is shown on one drawing, the parts share the drawing number and get individual dash numbers, such as 40267-1 and 40267-2. Dash numbers also separate left-hand and right-hand parts, and "some prints have odd numbers for left-hand parts and even numbers for right-hand parts." The AD above follows that pattern: 740-4000-501 and -503 are left-side doors, while -502 through -508 are right-side.

For standard hardware, the same handbook says that NAS dash numbers and suffix letters "do not have standard meanings" — they separate sizes, plating, drilling and so on, and the specific standard sheet gives the legend. Manufacturers' own systems vary as well. Airbus's published description of its part numbering puts a two-digit technical specification suffix after the basic number to describe a spares delivery configuration, such as holes left undrilled or hardware included. Parts with the same basic number are interchangeable "provided STS differences are incorporated during part replacement." Even there, a suffix can mean the same part with work still to do.

So treat a different dash number or suffix as a different part until your IPC says otherwise, in the direction you need. Neighboring dash numbers can be a left-hand and a right-hand part, two sizes, or two configurations of the same assembly.

Punctuation is a separate trap from real dash-number differences. Most variants are formatting, but the text alone does not tell you which characters the manufacturer treats as part of the number. Compare an offered number with the IPC entry, not with another quote.

"Alternate" on a quote vs an approved alternate

Suppliers offer alternates routinely, and the decision stays with the buyer. As an industry guide to reading quotes puts it, alternate part numbers "may be offered in response to a request for a quote," and "it is up to the operator to decide if the alternate part number can be accepted."

Read that as roughly one quote line in forty on which the supplier proposed something other than what was asked, each one a check against the IPC before it becomes a purchase order. It does not mean 2.5% of parts are interchangeable. Note too that "equivalent" and "substitute" do not appear among the parts list notations above: until your documents say the same, they are the supplier's opinion.

Why an NSN is not an IPC alternate

A National Stock Number (NSN) is, in the Defense Logistics Agency's words, the "official label applied to an item of supply that is repeatedly used throughout the Federal Supply System." Its 13 digits are a four-digit federal supply class, two digits for the country that assigned it, and seven digits unique to the item.

Take 5310-00-807-1474. Class 5310 is nuts and washers, and the item is a self-locking, extended-washer hexagon nut. A public NSN lookup lists more than twenty reference part numbers under it, from the military standard number MS21042L3 to several manufacturers' own numbers. They share a stock number because the federal supply system catalogs them as one item of supply. That grouping serves government supply. It is not your type certificate holder's statement about what may be installed on your aircraft.

The FAA's position on surplus parts points the same way. AC 20-62E allows parts from surplus sources "provided it is established that they meet the standards to which they were manufactured, interchangeability with the original part can be established, and they are in compliance with all applicable ADs." When a line says "ALT NSN …", the question is unchanged: is the part number actually offered listed for that position in your IPC?

What your engineering has to approve

Under FAA rules, the continued airworthiness of an aircraft, replacement of parts included, is the owner or operator's responsibility, and the installer must determine that a part is eligible for installation before returning the aircraft to service (AC 20-62E). An installation that amounts to a major repair or alteration needs FAA-approved data. In practice, offers sort into three cases:

PMA parts have their own approval basis and eligibility; see our guide to PMA parts vs OEM.

How to write the RFQ so alternates are quoted — or excluded

1. Write the number as your IPC prints it. Base number, dash number and suffix, exactly.
2. Add an "Alternates acceptable" column, Y or N per line. For Y, list the numbers your IPC allows, or write "IPC alternates only".
3. Ask for each alternate as its own quote line. Its own part number, condition, certificate, trace, quantity, price and lead time. An alternate buried in a note on the requested line cannot be compared, ordered or received cleanly.
4. Ask for the relationship and its source. The IPC figure and item, the SB number, or the manufacturer's notice. "ALT" on its own is not a source.
5. Say what happens otherwise. "Lines marked N: quote the exact part number only; offers on other numbers will not be considered."

When a quote comes back on a different number, run through the same short list every time:

• Is it a part number, or a stock number or catalog reference?
• Is it listed for that position and effectivity in your IPC, and with which notation?
• Which direction does the interchangeability run, and which configuration is your aircraft or unit in?
• Does any SB you have embodied, or any AD, rule it out?
• Will the part marking and the release certificate carry the number you actually order — the alternate's, not the one you asked for?

More on building the rest of the request in our guide to writing an aircraft parts RFQ, and on comparing what comes back in why one part number gets very different prices.

Sources

• Component Maintenance Manual 26-20-02, Portable Water-Solution Fire Extinguisher (M892480), Illustrated Parts List introduction — Mass Systems (AMETEK), revision 5, May 2003.
• AD 2017-13-10, Airworthiness Directives; Airbus Airplanes (engine fan cowl doors) — FAA, 82 FR 29371, 29 June 2017.
• FAA-H-8083-30B, Aviation Maintenance Technician Handbook – General — FAA, 2023 (reference and dash numbers; NAS part numbers).
• Airbus Part Numbering System — Airbus, AirbusDoc material management pages.
• Order 8110.42D (Change 2), Parts Manufacturer Approval Procedures — FAA, 27 October 2023 (IPCs "are not FAA-approved").
• AC 20-62E (Change 1), Eligibility, Quality, and Identification of Aeronautical Replacement Parts — FAA, 2010, change 2018.
• National Stock Number (NSN): Your Key to Unlock Sales — Defense Logistics Agency, 2011 DLA Industry Conference.
• NSN 5310-00-807-1474 — public NSN lookup (item name and reference part numbers).
• Part interchangeability — IFS Maintenix documentation.
• What To Know When Reading Aircraft Part Quotes — C&L Aero.